Process for preparing lignosulfonates

ABSTRACT

Disclosed herein is a process for preparing sulfonated lignins by selectively isolating lignins having molecular weights greater than 5,000 and subsequently reacting the lignin with sodium sulfite and an aldehyde. The resulting sulfonated lignins are useful as dispersants for disperse dyes and vat dyes.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a new and improved process for preparingsulfonated lignins and more particularly to a process for preparingsulfonated lignins of low viscosity possessing improved heat stabilityproperties.

(2) The Prior Art

Dyestuff compositions generally comprise a dye cake, i.e., of dispersedyes or vat dyes, and a dispersant. These dyestuff compositions arewidely used to color both natural and synthetic fibers. In the dyestuffcomposition, the dispersant serves three basic functions: (1) it assistsin reducing the dye particle to a fine size; (2) it maintains adispersing medium; and (3) it is used as a diluent.

Dye dispersants are generally one of two major types, sulfonated ligninsfrom the wood pulping industry via the sulfite or kraft processes ornapthalene sulfonates from the petroleum industry.

The advantages of employing sulfonated lignins as dispersants indyestuff compositions is based on their unique physical properties whichinclude good compatibility to many dye systems, outstanding dispersantcharacteristics at ambient and elevated temperatures and availability.There are, however, a number of disadvantages in employing lignins,whether they are sulfite lignins or sulfonated kraft lignins, asdispersants. These negative factors relate to fiber staining, heatstability and viscosity of the lignins employed. These adverseproperties are troublesome to dyers and many attempts have been made toovercome these disadvantages.

A number of technological developments have resulted in new methods andprocesses to modify sulfonated lignins to reduce the negative aspects ofemploying such materials as dye dispersants without simultaneouslycausing any major adverse effects upon those properties which rendersulfonated lignins desirable as dyestuff dispersants. U.S. Pat. No.4,001,202, which names as a co-inventor the sole inventor in thisapplication, describes a process for preparing a sulfonated lignin withimproved fiber staining properties useful as a dye dispersant, byreacting such lignin with an epihalohydrin. Also, U.S. Pat. No.4,338,091, issued to the inventor in this application, teaches reactinga modified lignin with sodium sulfite and an aldehyde. The lignin ismodified, however, by a pretreatment with sodium dithionate; thus, thesubsequent reaction does not involve the lignin material of the presentinvention.

Additional examples of reacting or modifying lignins to make them moresuitable as dye dispersants include U.S. Pat. Nos. 4,184,845, 4,131,564,3,156,520, 3,094,515, 3,726,850, 2,680,113, and 3,769,272. The art citedis meant to show the state of the art and is not intended to be allinclusive of lignin modifications.

Although the methods for treating and preparing sulfonated ligninsdescribed above offer some advantage during dyeing, none have produced aproduct possessing the improvements obtained by the products madeaccording to the claimed process.

It is the general object of this invention to provide a process wherebysulfonated lignins or lignosulfonates may be prepared to improve theirusefulness as dye dispersants.

Another object of this invention is to provide a process for reducingthe viscosity of sulfonated lignins or lignosulfonates.

Another object of this invention is to provide a process for improvingthe heat stability of sulfonated lignins or lignosulfonates.

Other objects, features and advantages of this invention will be seen inthe following detailed description of the invention.

SUMMARY OF THE INVENTION

It has been found that the viscosity of sulfonated lignins useful as dyedispersants in dyestuff compositions can be reduced with improvement inthe heat stability properties of the lignin by removing the lowmolecular weight component of the lignin and reacting the resultinglignin material with sodium sulfite (Na₂ SO₃) and an aldehyde at a lowpH.

DETAILED DESCRIPTION OF THE INVENTION

Lignin is obtained from spent pulping liquors, known as black liquor, ofthe pulping industry where lignocellulosic materials, such as wood,straw, corn stalks, bagasse, and the like, are processed to separate thecellulose or pulp from the lignin. The lignins employed in the processof this invention are obtained, preferably, from the kraft wood pulpingprocess wherein the natural lignin is present as a sodium salt. In kraftpulping, the wood is subjected to the effects of strong alkali. Thelignin in this process forms a soluble sodium salt which is separatedfrom the cellulose and dissolves in the pulping liquor. The lignin isrecovered from the spent pulping liquor, known as black liquor, byacidification.

Acidification of the black liquor containing the lignin salt isaccomplished by the introduction of carbon dioxide. Upon the addition ofcarbon dioxide to the black liquor, the phenolic hydroxide groups on thelignin molecule, which are in ionized form, are converted into theirfree phenolic or acidic form. This conversion renders the lignininsoluble in the black liquor as a result of which it precipitates out.

The lignins present in black liquor exist at various molecular weights.Lignins with a molecular weight of about 5,000 or less have been foundto negatively affect the heat stability of any subsequently sulfonatedlignin materials. In the practice of the present invention, theacidification of the alkaline black liquor containing the lignin salttakes place at a pH where all but a negligible amount of the ligninshaving a molecular weight of about 5,000 or less do not precipitate outwith the higher molecular weight lignin products, and, therefore, remainin solution with the black liquor. Generally, this selectiveacidification of black liquor to remove the lignins should occur at a pHranging from about 9.7 to about 11.0 to minimize the amount of ligninsremoved having molecular weights of 5,000 or less. Preferably, theacidification should occur at a pH ranging from about 10.0 to about11.0, such that essentially no lignins having molecular weights of 5,000or less are removed from the black liquor.

The alkali lignins are usually recovered from black liquor as waterinsoluble products by this precipitation procedure. Lignin obtained fromthe kraft, soda or other alkaline processes, to which this invention isdirected, is not recovered as a sulfonated product, but is easilysulfonated by reacting such material with a bisulfite or sulfite. Asulfonated lignin is any lignin containing at least an effective amountof sulfonated groups to give water solubility in moderately acid andhigher pH solutions.

The next step in developing a lignin-based dye dispersant is to properlysulfonate the lignin. It should be noted that the degree of sulfonationof a lignin is proportional to the solubility of that lignin in anaqueous solution and the viscosity of such lignin.

One of the conventional processes for sulfonating a lignin involves thesulfomethylation of alkali lignin by reacting such lignin with sodiumsulfite and formaldehyde. This process is mentioned by E. Adler et al.in U.S. Pat. No. 2,680,113. Sulfomethylation acts upon the aromaticnuclei of the lignin molecule in such a manner that --CH₂ SO₃ H groupsare bonded to such nuclei. Adler teaches that the treatment of thelignin with these sulfonation agents is carried out within a temperaturerange of 50° C. to 200° C., suitably 80° C. to 170° C., preferably 100°C. to 160° C. The quantity of sulfite used, calculated as anhydroussodium sulfite, may vary from about 10% to 100% of the quantity ofanhydrous lignin and the quantity of aldehyde is equivalent to thequantity of sulfite or lower, down to about 1%, calculated on thequantity of the anhydrous lignin material. The treatment is preferablycarried out in an alkaline solution.

In the practice of the present invention, the alkali lignin is mixedwith water to form a slurry. To the slurry the sulfomethylation agentsare added, i.e., sodium sulfite and formaldehyde. The ratio of sodiumsulfite to formaldehyde ranges from about 1.1:0.1 to about 2.5:1.0, thepreferred range being about 1.3:0.8. It has been found that the additionof excess amounts of sodium sulfite as compared to formaldehyde yields asulfonated lignin product with reduced molecular weight.

When formaldehyde and sodium sulfite are combined in stoichiometricamounts, they form, almost instantaneously a hydroxy methanesulfonate,which in turn undergoes reaction with the lignin to form sulfonatedlignin derivatives. The hydroxy methanesulfonate intermediate exists toonly 90% in the desired form, while 10% of the reactants are present atall times. A problem with having 10% of the reactants present is thatformaldehyde undergoes coupling reactions with the lignin beingsulfomethylated. It would, therefore, be advantageous to have as smallan amount of unreacted formaldehyde in the reaction mixture as possible.This can be accomplished by varying the mole ratio of sodium sulfite toformaldehyde. Increasing the mole ratio of sodium sulfite toformaldehyde results in the generation of a greater amount of hydroxymethanesulfonate intermediate at the expense of remaining amounts ofunreacted formaldehyde. The increased quantities of sodium sulfiteresult in a lower degree of polymerization and consequentially asulfonated lignin with a lower molecular weight.

Under prior practices, the sodium sulfite and formaldehyde were added toa lignin solution having an initial pH of about 9.0. The addition of thesulfonation agents would raise the pH of the resulting sulfonated ligninsolution to an even higher level. In the practice of the presentinvention, the precipitated lignin is subjected to an acid washingprocedure, preferably with sulfuric acid, and dried to produce a ligninmaterial having a pH ranging from about 1.5 to about 5.0. The lignin iscombined with water to form a slurry of about 25% solids. The ligninexists at this pH range in precipitated form. If the lignin slurry pH isbelow 5, the pH is adjusted to about 5.0 through the use of sodiumhydroxide. At this point, sodium sulfite is added to raise the initialpH of the reaction mixture to a range of about 7.0 to 7.5.

Sulfonation occurs with the addition of formaldehyde which raises the pHof the slurry to a range of about 8.0 to about 9.2. The slurry is thentaken to a temperature ranging from about 130° C. to about 175° C., thepreferable temperature being about 140° C. The temperature is maintainedfor a period of time ranging from about 30 minutes to 12 hours, the mostpreferable being about 2 hours.

The use of low pH and low temperature has two advantages. For one thing,the lignin is less likely to decompose at these conditions than underthe normal reaction conditions. The fact that the sulfonation occurs ata low pH means that the resulting sulfonated lignin product will possessa pH lower than would otherwise be obtainable.

When sulfonated lignins are employed as dye dispersants, it is preferredthat the pH of such lignin range from about 4 to about 8. If a lignin issulfonated at a high pH, the resulting sulfonated lignin will have ahigh pH. In order for a dyer to use such sulfonated lignin as adispersant, the dyer is required to add an acid to such lignin to lowerthe pH which translates into additional expense. By sulfonating thelignin at a low pH, a sulfonated lignin is obtained which does notrequire the use of acid to render it suitable as a dye dispersant.

The elimination of the need to employ an acid to lower the pH ofsulfonated lignin, in order to use it as a dispersant, has an additionaladvantage besides cost. The use of an acid to lower the pH normallyresults in the production of an electrolyte. The presence of anelectrolyte in the sulfonated lignin has a tendency to affect in anegative manner the heat stability of certain dye products which containsuch lignin as a dispersant. The elimination of the need to add acid tothe sulfonated lignin avoids the problems associated with the presenceof electrolytes.

The sulfonated lignins prepared in accordance with the present inventioncan be employed as dispersants in dyestuff compositions. The amount ofsuch dispersant required will vary depending upon the particular dyecake, the material to be dyed and the effect desired. Amounts up to 75%of the dispersant, based upon the weight of dried cake, may be used. Themost important factor in determining the proper amount of dispersantcomposition to be used in making up the dyestuff is the particular dyecake used. Generally, this amount will vary from dye to dye.

The following are examples only and should not be construed as limitingthe invention.

EXAMPLE 1

Lignin was isolated from spent pulping liquors from the kraft woodpulping process through acidification of the pulping liquors undervarious pH conditions as indicated in Table I. The isolated lignin wasacid-washed with sulfuric acid to a pH of about 5.0. A 25% solids slurryof lignin was prepared by admixing the recovered lignin with water.Sodium sulfite was added to the slurry at a pH of about 5.0.Formaldehyde followed the addition of sodium sulfite in close sequence.Upon addition of both sulfomethylation ingredients, the pH of thereaction mixture was 8.6-9.2 and the temperature was adjusted to 70° C.After 1 hour the temperature was raised to 140° C. and maintained for 2hours.

The viscosities of the sulfonated lignin product resulting from each ofthe samples were determined and are reported in Table I. The heatstability properties of the sulfonated lignin products resulting fromeach of the samples were also determined and appear in Table I.

Viscosity was measured by heating the sulfonated lignin solution toabout 70° C. and adding glacial acetic acid slowly until a pH of 8 wasobtained. The solids concentration was adjusted to 25%. A BrookfieldViscometer (model LVT) was used for all measurements. Measurements tookplace at 25° C.

Heat stability was determined by preparing a dye composition containingthe sulfonated lignin. The dye composition was prepared by mixing 50grams of Red I, 35 grams of sulfonated lignin, 125 milliliters of waterand 5 drops of EDTA (1% solids at pH of from 8.6 to 9.2). The pH wasadjusted to 8 with acetic acid or sulfuric acid. The dye composition wasground in a ball mill to the point where the filter test for dispersedyes was passed.

Heat stability was measured by adding 1 gram of the solid dyecomposition to 250 milliliters of water. The solution was boiled for 15minutes and then filtered through a tared Whatman filter paper No. 2above a No. 4 (with vacuum) as specified in the standard AmericanAssociation of Textile Chemists and Colorists (AATCC) heat stabilitytest. The filter paper was dried, and the residue dye material remainingon the filter was calculated.

The data in Table I clearly indicates that the pH at which lignin isisolated from black liquor has a substantial effect on the sulfonatedlignins subsequently produced. Improvements in viscosity and the heatstability of the sulfonated lignins are notable when raising the pH atwhich black liquor is isolated.

                  TABLE I                                                         ______________________________________                                                                                Heat                                  Lignin   Mole                           Stability                             Isolation                                                                              Ratio                  Viscosity                                                                             Filter                                pH from  Na.sub.2 SO.sub.3                                                                      Starting Final                                                                              at pH   Residue                               Black Liquor                                                                           CH.sub.2 O                                                                             pH       pH   25% Solids                                                                            (mg)                                  ______________________________________                                        9.2      1.3/0.8  8.7      8.90 8,500.0 128.0                                 9.7      1.3/0.8  8.7      8.95 117.5   60.0                                  10.1     1.3/0.8  8.6      8.63 27.0    18.5                                  ______________________________________                                    

EXAMPLE 2

Lignin was isolated from spent pulping liquors from the kraft woodpulping process through acidification of the pulping liquors at a pH ofabout 9.8. The isolated lignin was acid-washed with sulfuric acid to apH of about 5.0. A 25% solids slurry of lignin was prepared by admixingthe recovered lignin with water. Sodium sulfite was added to the slurryat a pH of about 5.0. Formaldehyde followed the addition of sodiumsulfite in close sequence. Upon addition of both sulfomethylationingredients, the pH of the reaction mixture was adjusted to variouslevels indicated in Table II. The temperature of the reaction mixturewas adjusted to 70° C. and maintained at such condition for 1 hour afterwhich the temperature was raised to 140° C. and maintained for 2 hours.

The viscosity of the sulfonated lignin product resulting from each ofthe samples was determined and is reported in Table II. Viscosity wasmeasured in accordance with the procedure outlined in Example 1.

The heat stability of the sulfonated lignin product resulting from eachof the samples was determined and is reported in Table II. Heatstability was measured in accordance with the procedure outlined inExample 1.

The data in Table II clearly indicate that the pH of which thesulfonation of lignin occurs in conjunction with the mole ratio ofsulfonating agents has a substantial effect on the sulfonated ligninssubsequently produced. Improvements in viscosity and the heat stabilityof the sulfonated lignins are notable.

                  TABLE II                                                        ______________________________________                                                                            Heat                                      Mole                                Stability                                 Ratio                      Viscosity                                                                              Filter                                    Na.sub.2 SO.sub.3                                                                      Starting Final    at pH    Residue                                   CH.sub.2 O                                                                             pH       pH       25% Solids                                                                             (mg)                                      ______________________________________                                        1.0/1.0  10.3     10.40    400,000  38.0*                                     1.0/1.0  9.9      10.65    400,000  18.5                                      1.3/0.8  8.6      8.70     27       18.0                                      1.4/0.7  8.9      9.25     23       26.7                                      1.5/0.5  9.0      9.40     22       103.0                                      2.0/1.15                                                                              9.6      9.80     24       302.0                                     ______________________________________                                         *Lignin was manufactured at 100° C. instead of 140° C.     

While the foregoing examples have illustrated an embodiment of theinvention, it should be understood that other variations andmodifications falling within the scope of the appended claims are to beincluded therein.

What is claimed is:
 1. In a process for preparing sulfonated ligninswhich comprises reacting lignin material with an aldehyde and a sulfite,selected from the group consisting of water soluble sulfites andbisulfites, wherein the improvement comprises the steps of(a)selectively isolating the lignin material from alkaline black liquor byreducing the black liquor pH such that all but a negligible amount ofthe lignins having a molecular weight of 5,000 or less remain insolution with the black liquor, (b) acidifying the lignin material to apH of from about 1.5 to about 5.0 by washing with sulfuric acid followedby adding water to the lignin material to form a slurry of about 25%solids, (c) adding to the lignin slurry the sulfite and the aldehyde ina sulfite to aldehyde mole ratio of from about 1.1:0.1 to about 2.5:1.0,which addition produces an initial reaction pH of from about 8.0 toabout 9.2, and (d) the temperature of reaction is raised from ambient tofrom about 130° C. to about 175° C. for from about 0.5 to about 12hours.
 2. The process of claim 1 wherein the lignin material is isolatedfrom the black liquor at a pH ranging from about 9.7 to about 11.0. 3.The process of claim 2 wherein the pH range is from about 10.0 to about11.0.
 4. The process of claim 1 wherein the water soluble sulfite issodium sulfite and the aldehyde is formaldehyde.
 5. The process of claim4 wherein the mole ratio of sodium sulfite to formaldehyde is about1.3:0.8.
 6. The process of claim 1 wherein the initial reaction pH isabout 8.6.
 7. The process of claim 1 wherein the addition of sulfite andaldehyde is sequential in that order.
 8. The process of claim 1 whereinthe reaction temperature is first raised to 70° C. for 1 hour after step(c) and before step (d).
 9. The process of claim 8 wherein the reactiontemperature of step (d) is 140° C. and said temperature is maintainedfor 2 hours.
 10. The process of claim 1 wherein the lignin slurry pHbelow 5.0 is raised to 5.0 by addition of sodium hydroxide.